It is advantageous to produce metal boxes for electrical interconnections with knockouts of two different sizes so that the inventory of boxes can be minimized and so that a box which is already installed has the flexibility of being connectible to either of two sizes of conduit. In this context, the term "electrical box" is used to include substantially any kind of metal box designed for housing the interconnection of wires from various cables or the mounting of electrical devices. The term includes relatively large housings such as metal cabinets as well as smaller housings such as switch boxes, masonry boxes, outlet boxes, extension rings and the like. It is known to produce concentric knockouts in several ways as shown by the following patents:
U.S. Pat. No. 2,850,916, Kingdon; PA1 U.S. Pat. No. 3,333,501, Pitcher; PA1 U.S. Pat. No. 3,587,906, Pepe; PA1 U.S. Pat. No. 3,877,601, Evans et al.
There are, however, problems with concentric knockouts having to do with proper grounding of the conduit to the box and also having to do with the ability of the electrician to remove the knockouts.
As to the grounding, if a box is made with concentric knockouts which are selectively removable to accommodate either 1/2" or 3/4" conduit, the knockout ring for the 3/4" conduit is about the same size as the lock nut for the 1/2" conduit. The ring is usually outwardly offset from the plane of the box wall. Thus, the lock nut engages only the ring and makes little if any contact with the box itself. Furthermore, the ring is usually attached to the box by two narrow bridging members which are not acceptable by the National Electrical Code or U.L. as being adequate grounding means between the box and conduit. It is therefore necessary to provide separate bonding jumpers between the 1/2" conduit and the box in order to satisfy safety and code grounding requirements.
The second problem arises from the same structural circumstances but is a mechanical rather than electrical problem. When a 1/2" conduit is mounted in the remaining 3/4" ring of a concentric knockout arrangement, the lock nut and conduit engage only the ring, as described above. Thus, any lateral mechanical forces exerted on the conduit are transferred directly to the ring. It is therefore necessary to use at least two bridging members between the ring and the box in order to provide mechanical stability. This, however, makes the ring extremely difficult to remove when it it necessary to attach a 3/4" conduit to the box.
For these reasons, concentric knockouts have not been acceptable, despite their apparent advantages. An eccentric knockout arrangement is shown in U.S. Pat. No. 2,154,310, Kinnard, but does not overcome the problems associated with the concentric knockouts discussed above and presents additional manufacturing problems.